The invention relates to an apparatus and method for providing anti-lock brake control. In particular, the invention relates to an apparatus and method for dumping and applying brake pressure at particular rates based on whether a wheel is in a stable or unstable condition.
Pressure control systems are necessary in order to control an amount of brake pressure to be applied and to be dumped from brake systems. Pressure control is used for electro-hydraulic brake (EHB) systems, which can also perform the control functions used for anti-lock brake systems (ABS), traction control systems (TC), and vehicle stability control systems (VSC).
A block diagram of a conventional braking system for a vehicle is shown in FIG. 1. Such a system is described in U.S. Pat. No. 5,551,769, issued to Mark Luckevich, and which is incorporated herein by reference. The conventional braking system is a hydraulic system, and includes a brake pedal 212, a brake switch 213, a brake master cylinder 214, control valves 216, brake fluid conduits 218, 220, 222, 224 and 226, as well as a right front wheel 228 and a left front wheel 230, and a pair of rear wheels 232.
The conventional system includes a pair of vehicle wheel speed sensors 234 for measuring the speed of each of the front wheels 228 and 230, and a pair of vehicle wheel speed sensors 236 for measuring the speed of each of the rear wheels 232. The system further includes an anti-lock brake system control unit 238. Each of the wheel sensors 234 and 236 are operatively connected to the control unit 238, which is itself operatively connected to the anti-lock brake system control valve 216, via an electrical or fiber optic connection.
In conventional braking systems, such as that shown in FIG. 1, the driver input is through the brake pedal 212 to the master cylinder 214. This is a mechanical system in which the brake command and power is provided by the mechanical motion of a master cylinder piston acting on the brake fluid. The pressure is then boosted (usually with a vacuum system) and actuated through the fluid to the brake calipers and rotors.
Conventional anti-lock brake systems determine a reference vehicle speed based on the measured wheel speeds for each wheel of the vehicle. The measured wheel speeds can be determined by several different methods. One method, for example, is to count a number of pulses for a given period of time, where each pulse corresponds to a predetermined position on a wheel passing through a predetermined reference location. Based on the measured wheel speeds for each wheel and the estimated vehicle speed, it can be determined if any of the wheels are departing from the estimated vehicle speed, thereby signifying a potential wheel slippage condition.
That is, when one applies brakes on a road surface in which some or all of the wheels are slipping excessively (i.e., traveling at slower than the vehicle speed), the anti-lock brake system should activate, in which case the brakes are pumped at a very fast rate so as to provide controlled braking.
FIGS. 2A-2C show a condition in which brake pressure is applied and then dumped, based on the difference between the wheel speed and the estimated vehicle speed. In each of FIGS. 2A-2C, the x-axis corresponds to the same passage of time. During normal conditions, the wheel speed 210 for each wheel should closely approximate the wheel speed reference 220. However, during braking, when the applied brake force exceeds the adhesion capabilities of the road surface, some or all of the wheels may lock, causing the measured wheel speed 210 for the locked wheel to depart from the wheel speed reference 220 by more than a fixed value .alpha.. That "departure" is shown as area 201 in FIG. 2A, and when the departure occurs, an ABS.sub.-- ACTIVE flag 225 is set to an active (high) state, as shown in FIG. 2B, so as to enter the ABS mode.
During the departure period (area 201), the brake pressure for each wheel experiencing a departure has to be dumped, in order to lessen the amount of brake pressure applied at that time. The lessening of the amount of brake pressure is performed in a step-wise fashion, where the width of the steps and the height of the steps can be adjusted.
Due to the dumping of the brake pressure, at some later point in time, the measured wheel speed should closely match the estimated vehicle speed, assuming the anti-lock braking system is functioning properly. During that period, shown as area 203 in FIG. 2A, the brake pressure is reapplied in a stepwise fashion, where the size and width of the steps are adjustable. The stepwise brake pressure apply and stepwise brake pressure dump are shown in FIG. 2C, for a conventional anti-lock brake system.
In FIG. 2C, P.sub.M/C 230 corresponds to a desired brake pressure of the operator (e.g., the amount of travel of the brake pedal and/or the amount of force applied to the brake pedal), and P.sub.brake 240 corresponds to an actual brake pressure at the brakes. P.sub.brake 240 becomes different from P.sub.M/C 230 due to the activation of the ABS system, in which dumping and applying of brake pressure in order to achieve a controlled stop is effected.
In conventional systems, the height and width of the dump pulses (i.e., the amount of dumping for a given period of time) is determined by the wheel departure characteristics. An open loop control is used in which dump pulses occur when a threshold .alpha. is passed and continue until the wheel begins to accelerate out of the departure. The pulse width and pulse frame (or duty cycle) are often modified based on the wheel departure characteristics. The brake pressure is reapplied in a stepwise manner as long as the operator is applying the brakes. However, if the measured wheel speed departs again from the estimated vehicle speed, another dumping of brake pressure will take place, followed by another apply cycle. This process may continue for as many cycles as needed, until the vehicle braking has been stabilized or until the operator has let off of the brake pedal.
With the above-mentioned conventional brake apply and dump system, during the dump period, the dumping is based on the amount of difference of the measured wheel speed with respect to the estimated wheel or vehicle speed, and other parameters derived from the wheel speed in order to optimize the rate of dumping.